Conventional autothermal reformers introduce reactants (fuel, oxidants, steam, etc.) into the front of the reformer and allow the associated reactions to occur to completion as the reactants flow through the reformer. The fuel can come in a variety of forms, such as methanol, gasoline, ethanol, etc. The oxidant is typically provided in the form of oxygen (O2) or air (O2 mixed with N2). The steam is typically superheated steam which supplies heat and water to the reformer. The superheated steam can be mixed with the oxidant flow prior to entering the reformer. The reformer converts these reactants into hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2), and water (H2O). The reformer can be used as a fuel source for supplying H2 to a fuel cell that uses the H2 as an anode reactant that, in conjunction with a cathode reactant produces electricity. The reformer can also be used to produce H2 that is then stored until the H2 is needed, such as at an H2 production facility.
During the startup of the reformer, and of the system within which the reformer is operating, superheated steam may not be readily available. Therefore, during startup the typical reformer will use the fuel and the oxidant in conjunction with a catalyst to perform a catalytic partial oxidation reaction until superheated steam is available. For example, when methane is used as the fuel, the catalytic partial oxidation reaction is as follows:CH4+½O2→CO+2H2, which has a ΔH=−247 KJmol−1.
As can be seen, the catalytic partial oxidation is an exothermic reaction that generates a large amount of heat. As a result, the large amount of heat generated by the catalytic partial oxidation reaction can cause damage to the components within the reformer. The catalytic partial oxidation reaction can also cause hot spots to occur within the catalyst beds or within the reformer that can damage the catalyst bed and/or the reformer. Therefore, it is desirable to control the temperature within the reformer as a result of the catalytic partial oxidation reaction.
Another concern during the startup of the reformer is the formation of carbon within the reformer. Carbon formation in the reformer can quickly plug the catalyst bed and cause additional damage to the reformer. Carbon formation occurs when there is insufficient oxidant for the carbon molecule to bind with during the catalytic partial oxidation reaction. For example, when there is an insufficient amount of oxidant, and the temperature is high, a pyrolysis reaction can occur according to the following formula:CH4→C+2H2, which has a ΔH=75 KJmol−1.
A disproportionation of CO can also occur in the form of a Boudouard reaction. The Boudouard reaction is as follows:2CO→C+CO2.The Boudouard reaction is catalyzed by metal, such as nickel, and, therefore, there is a high risk of the Boudouard reaction occurring in a reformer that uses a nickel or nickel containing catalyst or a nickel containing stainless steel in the reformer vessel. The risk of carbon formation is greatest during the startup because once steam is available and is added to reformer, the steam promotes the following steam reforming and shift reactions:CH4+H2O→CO+3 H2, with a ΔH=250 KJmol−1,CO+H2O→CO2+H2, with a ΔH=−41 KJmol−1,which has the effect of reducing the partial pressure of carbon monoxide in the fuel gas stream. Steam also leads to the carbon gasification reaction, as follows:C+H2O→CO+H2.
Therefore, the risk of carbon formation is greatest during the startup operation when steam is not available and/or limited. In order to avoid carbon formation, oxidant can be supplied in a ratio sufficient to avoid carbon formation due to the lack of steam. An oxygen to carbon ratio (O/C ratio) of at least 1.0 is needed to avoid carbon formation (if all carbon exits the reformer in the form of CO). Since some level of CO2 is favored by thermodynamic equilibrium, a higher O/C ratio is actually required, such as 1.1 or greater. However, such a high O/C ratio can cause a prohibitively high temperature in the reformer due to the catalytic partial oxidation reaction. The prohibitively high is most prone to occur at the front end of the reformer where the oxidant and fuel are fed into the reformer; however, prohibitively high temperature can be experienced throughout the length of the reformer.
Therefore, there exists a need to control the temperature that occurs within the reformer during startup while also providing an O/C ratio that is sufficient to prevent carbon formation within the reformer. Additionally, it is desirable to provide a reformer that has a more uniform temperature distribution along its length to increase the lifespan of the reformer and catalyst bed along with increasing the conversion of the fuel to H2.